Document Id::retain_autoreleased

Author: madsmtm

objc2/CHANGELOG.md

@@ -14,6 +14,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * Added convenience methods `Id::new_null`, `Id::as_ptr` and
   `Id::retain_null`.
 * The `objc2-encode` dependency is now exposed as `objc2::encode`.
+* `Id::retain_autoreleased` to make following Cocoas memory management rules
+  more efficient.
 
 
 ## 0.3.0-alpha.6 - 2022-01-03

objc2/src/rc/id.rs

@@ -216,6 +216,9 @@ impl<T: Message, O: Ownership> Id<T, O> {
     /// some API, and you would like to ensure that the object stays around
     /// so that you can work with it.
     ///
+    /// If said API is a normal Objective-C method, you probably want to use
+    /// [`Id::retain_autoreleased`] instead.
+    ///
     /// This is rarely used to construct owned [`Id`]s, see [`Id::new`] for
     /// that.
     ///
@@ -272,29 +275,49 @@ impl<T: Message, O: Ownership> Id<T, O> {
         NonNull::new(ptr).map(|ptr| unsafe { Id::retain(ptr) })
     }
 
-    /// TODO
+    /// Retains a previously autoreleased object pointer.
+    ///
+    /// This is useful when calling Objective-C methods that return
+    /// autoreleaed objects, see [Cocoa's Memory Management Policy][mmRules].
+    ///
+    /// This has exactly the same semantics as [`Id::retain`], except it can
+    /// sometimes avoid putting the object into the autorelease pool, possibly
+    /// yielding increased speed and reducing memory pressure.
+    ///
+    /// Note: This relies heavily on being inlined right after [`msg_send!`],
+    /// be careful not accidentally require instructions between these.
+    ///
+    /// [mmRules]: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html
+    ///
+    /// # Safety
+    ///
+    /// Same as [`Id::retain`].
     #[doc(alias = "objc_retainAutoreleasedReturnValue")]
-    // This relies heavily on being inlined right after `objc_msgSend`.
     #[inline(always)]
     pub unsafe fn retain_autoreleased(ptr: NonNull<T>) -> Id<T, O> {
         // Add magic nop instruction to participate in the fast autorelease
         // scheme.
         //
+        // See `callerAcceptsOptimizedReturn` in `objc-object.h`:
+        // https://github.com/apple-oss-distributions/objc4/blob/objc4-838/runtime/objc-object.h#L1209-L1377
+        //
         // We will unconditionally emit these instructions, even if they end
         // up being unused (for example because we're unlucky with inlining,
         // some other work is done between the objc_msgSend and this, or the
         // runtime version is too old to support it).
         //
-        // See `callerAcceptsOptimizedReturn` in `objc-object.h`:
-        // https://github.com/apple-oss-distributions/objc4/blob/objc4-838/runtime/objc-object.h#L1209-L1377
-        // and this StackOverflow answer for some background on why the design
-        // is like it is: https://stackoverflow.com/a/23765612.
-        //
         // It may seem like there should be a better way to do this, but
         // emitting raw assembly is exactly what Clang and Swift does:
         // swiftc: https://github.com/apple/swift/blob/swift-5.5.3-RELEASE/lib/IRGen/GenObjC.cpp#L148-L173
         // Clang: https://github.com/llvm/llvm-project/blob/889317d47b7f046cf0e68746da8f7f264582fb5b/clang/lib/CodeGen/CGObjC.cpp#L2339-L2373
         //
+        // Resources:
+        // - https://www.mikeash.com/pyblog/friday-qa-2011-09-30-automatic-reference-counting.html
+        // - https://www.galloway.me.uk/2012/02/how-does-objc_retainautoreleasedreturnvalue-work/
+        // - https://github.com/gfx-rs/metal-rs/issues/222
+        // - https://news.ycombinator.com/item?id=29311736
+        // - https://stackoverflow.com/a/23765612
+        //
         // SAFETY:
         // Based on https://doc.rust-lang.org/stable/reference/inline-assembly.html#rules-for-inline-assembly
         //